It is well known that when the size of a bulk material reduces down to nano scale, its original physical properties such as optical, electrical, magnetical, and mechanical properties would change dramatically. For example, the melting point of pure gold has a fixed value (about 1064° C.) but as the particle size reduces down to nano scale, there is no longer a fixed value. See Ph. Buffat and J. P. Borel et al., Size Effect of the Melting Temperature of Gold Particles, Phys. Rev. A, 1976, 13, 2287. According to the previous facts, it opens a field for nano material applications.
Nanometals have their applications in many fields. For example, many important catalysts are constituted by metals. Nano-scaled metals greatly enhance catalytical performance. For example, it is hard to apply a bulk of gold into a chemical reaction, but it can be used as an excellent catalyst for the oxidation of carbon monoxide at low temperature when its particle size is about 2 nm. In addition, the surface plasma resonance of nanometal demonstrates its unique and strong light absorption character, and it is affected by molecules adsorbed on the surface of the nanometal, and thus the nanometal can be used as a sensor. In medical engineering, nanometals can be used for the diagnosis, treatment, and prevention of diseases and in areas like drug delivery, medical detection, disease diagnosis, gene detection, etc. For example, molecules, like DNA or proteins, are attached onto nanometal particles and through the alternation of fluorescence, conductivity and magnetism, it would be used for the diagnosis and assistance of treatment. The high specific surface area of nanometal particles can enhance the sensitivity for the detection which would help detect the disease in early stages and therefore only damage cancer cells selectively. Among the variety of metals, gold has high biocompatibility and is particular being used in biolabeling and detection.
It is known that the property of a nanometal would change in accordance with its morphology and its application would also be affected by its morphology. For example, in medical engineering, it has been reported that as compared with gold nanospheres, gold nanoparticles with hexagon and boot shapes exhibit high-sensitivity surface-enhanced Raman scattering (SERS) and have been successfully applied to the detection of Avidin (an egg white protein). See Gold Nanoparticles with Special Shapes: Controlled Synthesis, Surface-enhanced Raman Scattering, and The Application in Biodetection, Sensors, 2007, 7, 3299-3311.
Currently, many methods for preparing a nanometal have been proposed, including laser ablation method, metal vapor synthesis, chemical reduction method, etc. In the laser ablation method, the high energy of laser is used to melt a metal, and through the low temperature environment provided by a solution and the stabilizing agent contained therein, the nanometal formed can be evenly dispersed in the solution. In metal vapor synthesis, the main principle is to atomize a metal into metal atom steam and then mix the metal atom steam with inert gas or organic steam. The steam is then condensed onto a clean surface at a low temperature, followed by a separation procedure to obtain a nanometal. In the chemical reduction method, the oxidized metal ion is reduced back into zero-charged metal by a reducing agent or an electrochemical system and the growth of a desired nanometal can be controlled with relevant operation conditions.
The chemical reduction method includes the commonly-used seed-mediated growth method. The principle of the seed-mediated growth method is based on using small-sized nanometals (usually in the range from a few nanometers into tens of nanometers) as a seed crystal and adding a reducing agent to allow the metal ion to be reduced and then grow to the desired size and morphological nanometal on the seed crystal. Seed-Mediated Synthesis of Gold Nanorods: Role of the Size and nature of the Seed (Anand Gole and Catherine J. Murphy et al., Chem. Mater., 2004, 16, 3633-3640) discloses a method for preparing nanometals via seed-mediated synthesis. It is necessary for such method to additionally formulate a growth solution for synthesizing gold nanorods with a step by step reaction. However, this method is extremely time-consuming, costly and is extremely complicated in procedure. Also, only a single-morphological nanometal would be obtained, and therefore, it is hard to promote such a method due to its low applicability.
Based on the above needs, this invention provides a simple preparation method that could obtain a multimorphological nanometal-containing nanometal dispersion with good dispersibility on a larger scale.